Hubble Reveals Origins of Hippocamp — The Tiny Moon of Neptune that Shouldn’t Be There

Hippocamp is a tiny moon born of planetary violence, according to research conducted using the Hubble Space Telescope (HST). This tiny natural satellite appears to have broken off, billions of years ago, from its larger companion, Proteus.

Astronomers using the Hubble Space Telescope discovered Hippocamp in 2013, but the discovery came with an intriguing question. The moon was found orbiting as close as 12,000 kilometers (7,500 miles) to the much-larger neighbor, Proteus. Such close proximity would normally throw the smaller satellite out of orbit of its host planet, or cause it to crash into the larger moon. This led astronomers to dub Hippocamp “The Moon that Shouldn’t be There.”

“The first thing we realised was that you wouldn’t expect to find such a tiny moon right next to Neptune’s biggest inner moon. In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now,” said Mark Showalter of the SETI Institute.

An artist’s impression of Hippocamp (foreground) orbiting Neptune, with Proteus in the background. This moon is too small and distant for surface details to be seen from Earth, even using the Hubble Space Telescope. Image credit: NASA, ESA and J. Olmsted (STScI)

Proteus is 418 kilometers (260 miles) in diameter, roughly the size of the state of Ohio. However, this dwarfs Hippocamp, measuring just 34 km (20 miles) from side to side, the size of a large city. It is the second-largest of the moons of Neptune, and the most distant from its planet of the inner moons.

Proteus, as seen by Voyager 2 in 1989. This is one of the darkest moons in the Solar System, reflecting just six percent of the light that strikes its surface. Image credit: NASA/JPL

In 1989, when Voyager 2 passed Neptune, the spacecraft spotted a large impact crater on Proteus, likely the result of a cometary impact in the distant past. Astronomers realized such a massive collision would have had nearly enough energy to shatter the moon. This new finding reveals the event chipped off a piece of Proteus, placing it into orbit around Neptune, as a new moon we see today as Hippocamp.

The moons of Neptune have a violent, chaotic past. Long ago, this planet captured a massive body from the Kupier Belt, which we see today as its largest moon, Triton. Gravity from this object, 2,700 km (nearly 1,700 miles) across, tore apart the early moons of Neptune. Slowly, debris from the first satellites coalesced, coming together to form the moons we see around the planet today. The formation of Hippocamp from a later cometary collision classifies this body as a third-generation satellite.

While most of the moons of Neptune orbit close to the planet, including Hippocamp and Proteus, mighty Triton draws a larger path around the world. Image credit: NASA, ESA, and A. Feild (STScI)

“Based on estimates of comet populations, we know that other moons in the outer solar system have been hit by comets, smashed apart, and re-accreted multiple times. This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets,” explains Jack Lissauer of NASA’s Ames Research Center.

Hippocamp was originally known as S/2004 N 1 when it was discovered, before being assigned its new name. The moniker derives from sea creatures of Greek and Roman origins, having the upper body of a horse and the lower body of a fish. The International Astronomical Union (IAU), the organization that supplies official names to objects in space, requires moons of Neptune to be named after aquatic beings in Greek and Roman mythology.

Along with Uranus, Neptune is classified as an ice giant. Despite the name, relatively little water ice is like found beneath its dense atmosphere. The core of the planet is thought to be made up of rock, surrounded by a dense ocean of water, methane, and ammonia. The atmosphere we see when we look at this world is composed mostly of hydrogen, helium, and methane.

So far, Voyager 2 is the only spacecraft to have ever visited this most-distant planet in the Solar System.